1. Field of the Invention
The present invention relates to the design of integrated circuit charge pumps, used to generate high positive or negative potentials, such as charge pumps utilized in integrated circuit flash EEPROM devices to establish program and erase potentials for the array.
2. Description of Related Art
In many integrated circuits, it is desirable to produce a potential higher than the standard supply voltage, or a voltage having a polarity opposite to the supply voltage. These potentials can be generated using charge pump circuitry implemented on the integrated circuit.
One application for charge pump circuitry is in the generation of program and erase potentials for flash EEPROM devices. Flash EEPROM devices need potentials on the order of 12-15 volts in order to program or erase the memory cells. Some devices resort to the use of a high programming potential supply external to the chip to provide these voltages. However, recent designs have migrated towards the use of standard 5 volt supplies only, relying on charge pumps to generate to programming and erasing potentials. These potentials may also be achieved by producing both negative and positive voltages using charge pumps, to avoid generating a high absolute value potential on chip.
One problem associated with charge pump circuitry is the requirement of capacitors on chip. One cost effective type of capacitor on integrated circuits, such as flash EEPROM, is based on connecting the source and drain of an MOS device together to provide one terminal of the capacitor, and using the gate of the device as the second terminal of the capacitor. These MOS capacitors are examples of "active" capacitors, which require a threshold voltage across the device to activate. Thus, the circuitry using these active capacitors must be properly biased during operation of the charge pump to ensure proper operation.
Another problem associated with charge pumps on integrated circuits is the high voltage produced. These high voltages can exceed the breakdown potentials of the active devices used to produce the capacitors. Thus, charge pumps have been developed using series capacitors. The voltage dividing action of series capacitors reduces the potential across any one of the capacitors in series. This allows the charge pumps to achieve higher voltages without reaching the breakdown threshold of the capacitors.
For instance, U.S. Pat. No. 5,008,799 invented by Montalvo, and U.S. Pat. No. 5,059,815 invented by Bill el al., describe charge pumps using series capacitors. The Bill et al. and Montalvo patents provide a discussion of the background to the present invention, and are incorporated by reference for such purpose.
In the Bill et al. patent, the series capacitors consist of a single active device in series with an active parallel plate capacitor. The active device in the series is biased to an active state by a pull up circuit. The parallel plate capacitor needs no biasing. However, parallel plate capacitors may provide undesirable design constraints in the implementation of the charge pumps on integrated circuits.
The Montalvo patent avoids the use of parallel plate capacitors on chip by using series active capacitors. In Montalvo, the second active capacitor (the output device) in the series is implemented using two "back-to-back" devices, for a total of three active capacitors in a single charge pump stage. The series active capacitor design presents a difficult biasing problem of maintaining a sufficient voltage at the common node between the capacitors to activate the input device while maintaining a sufficient voltage on the output node to activate the output device, in order to ensure that both capacitors remain activated during the charge pumping operation. Using the back-to-back capacitors ensures that at least one of the back-to-back capacitors is activated, except for brief transitions when the difference across the back-to-back capacitors is less than the threshold of either. During this transition, the pump capacitance is very small and the voltage dividing effect of the series capacitors is weakened.
It is desirable to provided a series capacitor charge pump which overcomes the problems of the prior art devices, and is suitable for use in integrated circuits.